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Positive to Earth & Negative Earth Voltage in a Healthy DC System

01/07/2012 3:06 AM

Hi friends,here is my query:What should be the ideal positive to earth & negative earth voltage in healthy 110V 0r 220v DC system.We have in our system,the measured values with respect to earth keeps changing.What can be assumed.?

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#1

Re: Positive to earth & negative earth voltage in a healty DC system

01/07/2012 6:27 AM

Is there any reference point to earth from either the + or - rails? If not you will get weird voltage readings to earth.

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#3
In reply to #1

Re: Positive to earth & negative earth voltage in a healty DC system

01/08/2012 8:17 AM

I think its a floating system...We have earth leakage ammeter connected to the charger panel.Its a 20 year old system.

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#8
In reply to #3

Re: Positive to earth & negative earth voltage in a healty DC system

12/12/2017 6:32 AM

<...its a floating system...> Then the voltage to earth doesn't matter; it's equivalent to monitoring the voltage between a car battery terminal and an earthed plate, for example.

<...earth leakage ammeter connected to the charger panel...> Then the voltage to earth will be the product of the current flowing through the ammeter and the resistance of the connection to earth. In which case <...ideal...healthy...> doesn't matter; if it did, then something would be done about it.

Next!

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#2

Re: Positive to Earth & Negative Earth Voltage in a Healthy DC System

01/07/2012 8:05 AM

As TonyS wrote, unless you have some definite connection to earth, you will get changing readings depending on the dirtiness and dampness of every wire, terminal and apparatus connected to the battery.

Each time something is switched on, it changes from being, say, all negative for a relay coil, to being connected to + and -. Consequently, the balance of its leakage is changed.

Usually, a 110 or 220V DC control battery will not be left floating - it will be earthed via an earth fault relay of high resistance. This will indicate when something gets earthed by a fault, before there are two earth faults and something gets switched when it should not.

In a well designed system, the resistance is too high to energise any control relay whose coil is earthed (but low enough compared to any normal insulation resistance that the sum of earth resistance is not changed more than a few %).

So first you must find if your system has such a relay. On an 110V/220V system, equal resistors to each pole from earth is often used, so voltage to earth is normally equal magnitude at + and - poles of battery. A voltage relay detects unbalance > xx volts and gives visual/audible alarm.

Some earth fault relays have a DC bias supply, which biases the positive of the battery negative - so anything connected to battery must always be negative to earth - the purpose being to reduce corrosion of conductors (especially fine coil wires) by any leakage current. This was the system for 50 volt telephone supplies.

So, do you have "Control Supply Earth Fault" or alarm of similar name?

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#4
In reply to #2

Re: Positive to Earth & Negative Earth Voltage in a Healthy DC System

01/08/2012 8:19 AM

Thanks buddy....its a old system & doesn't have earth fault relay.Only it has earth leakage ammeter.

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#5
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Re: Positive to Earth & Negative Earth Voltage in a Healthy DC System

01/08/2012 12:29 PM

Ammeter is good. It tells you the size of the leakage and perhaps its polarity.

My guess is that Ammeter is connected one terminal to earth and other terminal to junction of two equal-ohms resistors one to positive, other to negative pole of battery supply.

It will help you, if you find the value of the two equal resistors and the full scale of the ammeter - you will then know if the indicated current is enough to e.g. energize a control/protection relay without proper reason.

You can put a portable ammeter in series with the earth current meter to find its scaling.

Depending on sensitivity of ammeter, the readings may be fluctuating effect of damp on conductive dirt here and there (if it is very sensitive) or may be a certain device which has developed a high leakage.

If the changes you see are "step" between stopped and running, then it is likely you have one device with a leakage.

If it varies with "rain or not", but no clear event, since you say it is 20 years old, it may be leakage in the battery cells. Since the electrolyte is conductive when wet, any spillage or venting out over time will cause a conductive film on the cell case. Measure voltage to earth from each cell bank or row in turn, then go cell by cell. When you find the cell nearest earth, look around that cell and nearby for signs of leakage.

What you measure may be an assembly of leakage from many cells. In any event, you can try the effect of a 20000 ohm 10 watt resistor from each battery pole to earth on the voltage of that pole to earth, as an indication of the leakage resistance - but do not forget that the value of the earth current ammeter resistors will have a big influence on the effect of 20000 ohm.

If you have a general leakage of a few 1000 ohms from battery, you will have to wash every cell case sides with de-ionized/distilled water, to remove any electrolyte.

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#6

Re: Positive to Earth & Negative Earth Voltage in a Healthy DC System

01/09/2012 3:28 AM

If the voltage between supply terminals is constant and the voltage between terminal and earth varies, it indicates that a high impedance voltmeter is in use, and that the system is not connected to earth.

So the original question is unanswerable.

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#7

Re: Positive to Earth & Negative Earth Voltage in a Healthy DC System

01/10/2012 3:24 PM

commendable answers gurus.

thanks from my side too..

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